1. Field of the Invention
The present invention relates to a method of and a circuit for an automatic frequency control for stabilizing a receive frequency which is applied to a delay detection type radio communication system such as a digital cordless telephone system which uses a baseband signal essential to the delay detection.
2. Description of the Related Art
To stabilize a receive operation in a delay detection type radio communication system, automatic control of frequency is performed. An example of the automatic frequency control will be briefly explained by referring to FIG. 13.
FIG. 13 illustrates a demodulator of a conventional digital cordless telephone system having a delay detection function for a (.pi./4) shift QPSK (quadrature phase shift keying) signal. In the demodulator, a signal (.pi./4QPSK modulated signal) input through an input terminal 701 is mixed, by mixers 702 and 703, with frequency signals having .pi./2phase difference supplied from a local oscillator 704 through a .pi./2phase difference branching filter 705. Thus, the input signal from the input terminal 701 is subjected to an orthogonal modulation and the mixers 702 and 703 output baseband signals I and Q.
The baseband signals I and Q are converted into digital signals by A/D converters 706 and 707 and then transmitted to a delay detection circuit 710 for preforming delay detection via filters 708 and 709. An output of the delay detection circuit 710 is transmitted to a frequency error detection circuit 711. An output of the delay detection circuit 710 is also transmitted to the next stage of the cordless telephone system, such as a data decoding circuit.
The frequency error detection circuit 711 detects a frequency error from the output of the delay detection circuit 710. The detected frequency error is fed to the local oscillator 704 through a D/A converter 712 so as to control the oscillation frequency of the local oscillator 704, thereby performing the frequency correction of the receive signal.
Conventional radio communication systems typically perform the automatic frequency control (AFC) by controlling the frequency of a local oscillator of the receiving unit in the above-described manner.
It is to be noted that the local oscillator is generally controlled with an analog signal, and thus when a digital control signal is supplied, it is necessary to convert the digital signal into an analog signal by a D/A converter such as the D/A converter 712 of FIG. 13.
In the conventional AFC described above, it is necessary to always perform a fine step control to the local oscillator by an analog signal. Therefore, the AFC is disadvantageous in that (a) a circuit scale becomes large and (b) a production cost becomes high.
Further, in a multiplex system using the above-described step control method, since the response to frequency switching for each channel is slow, (c) time response is too slow to catch up with the operation of a time multiplex system such as a time division multiple access (TDMA) system and a time division duplex (TDD) system.
In order to eliminate these disadvantages, there is a known method in which a correction is performed to a signal obtained through delay detection by a digital signal processing circuit. Since this method does not require the step control, this method is suitable for realizing a fast AFC control required in the DTMA and TDD systems.
This method, however, has a disadvantage that, since the correction control must be carried out after filtering and delay detecting operations, (d) a desired spectrum is filtered out by the filtering and thus performance is remarkably degraded.